Detection of near-surface defects in viscoelastic material based on focused ultrasound thermal effects

Viscoelastic materials will absorb and dissipate energy under stress, resulting in energy loss and heat generation. The conventional non-destructive testing methods have certain limitations when it comes to detecting near-surface defects in viscoelastic materials. In this paper, a detection method of near-surface defects based on focused ultrasonic thermal effect is proposed. Firstly, the difference in thermal effects caused by defective and non-defective regions of the material under ultrasound is analyzed according to the stress response equation of viscoelastic materials, and the detection principle is elucidated. Secondly, the feasibility of this method is verified through finite element simulation with an example of plexiglass material Subsequently, the variations in the surface temperature distribution of defective specimens with varying diameters and depths are analyzed. Finally, experimental validation reveals that ultrasonic waves operating at 1.12 MHz successfully detect artificial defects with a diameter of 1 mm. With the increase of the equivalent diameter of the defect, the width of the low-temperature depression area in the surface temperature field exhibits a linear increase relationship. With the increase of the defect depth, the surface temperature difference between the corresponding position of the defective and the surrounding non-defective area gradually decreases. This method effectively overcomes the half-wavelength limitation and introduces a novel detection approach for near-surface defect identification in viscoelastic materials such as plexiglass.